There have been reported cases in drilling deep holes, such as for oil and gas, where the bit chuck has unscrewed from the down hole drill during operation, thus leaving the chuck end drill bit in the hole, often causing expensive problems.
The unscrewing of the chuck is believed to be caused by the extreme amount of torsional windup experienced in a 3000, 5000 foot drill string during the drilling operation and the drill string rapidly unwinding when the down hole drill is lifted from the bottom of the hole. The resulting rotation backwards of some 3 or more turns unscrews the chuck particularly in the case where the drill bit is partially stuck or perhaps once unloosened by continued reverse rotation. The loss of drill bit chucks may also be occasioned by the improper intentional reverse rotation of the drill string during stuck operation.